1. Field of the Invention
The present invention relates to an improved process for preparing an electrode for electrolysis.
2. Description of the Prior Art
Heretofore, the alkali metal electrolysis such as sodium chloride electrolysis has been mainly carried out by the mercury process. Recently, the pollution of drainage containing mercury component caused by the mercury process has been discussed. The change of the process from the mercury process to diaphragm processes has been required.
The diaphragm processes have been usually worked at higher pH in the electrolysis in comparison with the mercury process. The known electrodes have low oxygen overvoltage. Accordingly, when the known electrodes are used for the diaphragm process or the ion-exchange membrane process about 1 to 3% of oxygen is included in the resulting chlorine, whereby the anolytic gas can not be directly fed into petrochemical plants etc. It is necessary to use the anolytic gas after removing oxygen. Accordingly, special equipments and complicated operations are required which increase the cost.
In order to overcome the disadvantages, an electrode which causes a smaller generation of oxygen should be used. The electrode potential of oxygen at equilibrium (EO.sub.2) is lower than that of chlorine (E.sub.C12). When an electrode which does not have any selectivity in the electrode reaction of oxygen and chlorine is used, a large amount of oxygen is generated at the potential for generating chlorine.
Thus, in order to reduce the generation of oxygen, it is necessary to use an electrode having a coating which has the characteristic inhibiting oxygen electrode reaction in the theory of reaction rates.
The selectivity of the electrode for the electrode reaction is called an electrocatalytic activity which has been estimated by an exchange current density of the coating of the electrode.
It has been known that platinum group metals such as Ru, Pd, Rh, Pt and Ir have such electro-catalysis. The exchange current densities of these platinum group metals on the oxygen electrode reaction are as follows. EQU Ru&gt;Ir&gt;Rh&gt;Pd&gt;Pt.
The exchange current densities on the chlorine electrode reaction are as follows. EQU Pd&gt;Ru&gt;Ir&gt;Rh&gt;Pt.
From the viewpoints of smaller generation of oxygen and superior electrocatalytic activity on the chlorine electrode reaction, palladium is optimum.
However, in the practical use, when palladium is coated in the form of palladium metal, the palladium metal coating is dissolved in the electrolysis and it can not be practically used because of inferior anticorrosive property.
In order to overcome these disadvantages, it has been proposed to use anticorrosive electrodes made of a Pt-Pd alloy or prepared by coating the Pt-Pd alloy on a substrate or by oxidizing the surface of the Pt-Pd alloy. (B.P. 1,147,442, B.P. 1,195,871).
However, the electrocatalytic activity of palladium itself could not be imparted because the alloy of palladium is used and the anticorrosive property for a long time of the electrode is not satisfactory.
It has been proposed to use an electrode made of Pt-Pd alloy oxide. (B.P. 1,147,442, B.P. 984,973). In order to form the alloy oxide on a titanium substrate, it is necessary to treat it at high temperature in an atmosphere of oxygen under high pressure. In the treatment, the titanium substrate is severely oxidized and it is difficult to use it as the electrode. Accordingly, in the proposed method, the Pt-Pd alloy is coated on the titanium substrate and the alloy oxide is formed by anodic oxidation. The characteristics of the electrode are substantially the same with those of the electrode prepared by oxidizing the surface of the Pt-Pd alloy.
On the other hand, the inventors have studied to coat palladium oxide on a substrate made of titanium etc. However, the adhesiveness of the titanium substrate and the palladium oxide is not enough, and this approach has not been successful.
The inventors have further studied and have succeeded to obtain an electrode which can be practically used, by adding a small amount of another metal oxide to a large amount of palladium oxide to improve the mechanical strength. However, it has not been successful to decrease the consumption of the electrode to substantially zero.
The inventors have further studied the reason why the perfect anticorrosive property can not be attained by coating palladium oxide on the titanium substrate and have found that the corrosion is caused by a small amount of metallic palladium. That is, when titanium is directly contacted with palladium oxide or the unreacted palladium compound in the preparation of the palladium oxide coating on the titanium substrate in the thermal decomposing process, the palladium compound is reduced with titanium whereby metallic palladium is formed to contaminate the palladium oxide.
Accordingly, it is considered that the anticorrosive property is deteriorated by using the electrode having the improved mechanical strength for a long time because the metallic palladium formed by the reduction is dissolved in the electrolysis and the coated layer becomes porous and the coating falls down with the generation of the gas from the surface of the electrode.
The inventors have found from these facts that the consumption of the electrode can be completely prevented by removing the small amount of metallic palladium as the by-product by forming an alloy with platinum in the thermal decomposing process for forming the palladium oxide coating. The present invention has been attained by these findings.
The inventors have succeeded to improve the anticorrosive property in relatively high degree by forming an alloy by reacting platinum with the unreacted palladium. However, the shunt current is passed through the palladium platinum alloy in high current density whereby the dissolution of the alloy can not be prevented and the anticorrosive property for a long operation has not been satisfactory. The present invention is the improvement of our previous invention disclosed in the copending applications U.S. Patent Ser. No. 863,425, British Patent Application No. 54341/77 and West German Patent Application No. P2800193.6.